The price of commercial integrated circuits (ICs) (chips) is continuously under pressure but the cost of some testing may tend to persist if not increase. Test cost of HSIO may be even higher compared to conventional digital counter circuits because it may be more difficult to make an HSIO test work and may require additional specialized high-end automated test equipment (ATE). It is desirable to develop HSIO test circuits and processes that may speed up production test development and that may provide flexible test interface to accommodate a wide range of test applications. Such test circuits and processes may reduce overall costs and increase the value of products.
BISTs in HSIO commonly use a pseudo-random binary sequence (PRBS) generator in a transmitter circuit (TX) and a receiver circuit (RX). An example of a conventional PRBS BIST is shown in FIG. 1, in which capital letters Di denote an 8-bit data test vector and capital letters Ei denote a 10-bit codeword generated in a transmitter 12 (TX). The corresponding action of a receiver 14 (RX) is denoted using lower case letters di and ei. The stream of 8-bit data and of 10-bit codeword is enclosed in arrow brackets (> . . . >) indicating the beginning and end of test patterns. A PRBS generator 18 provides the sequence > . . . D3 D2 D1> which is encoded by encoder 22 to create the encoded sequence > . . . E3 E2 E1> to which control character(s) K0 (which may be a large number of control characters) is added to make a test pattern > . . . E3 E2 E1 K0>. The control character is used by the receiver to identify the beginning of an encoded test pattern. The encoded test pattern is serialized by serializer 24 and provided through channel 28 to deserializer 32 of RX 14. Deserializer 32 provides a received encoded test pattern > . . . e3 e2 e1 k0> to decoder 38 which provides the decoded received sequence > . . . d3 d2 d1>. The control character K0 may be removed before or after the test pattern is provided to decoder 38. A PRBS generator 44 in RX 14 generates the same sequence > . . . D3 D2 D1> as in TX 12 which is compared in block 48 with the decoded received sequence > . . . d3 d2 d1>. If the two sequences are identical, then there is not an error, if they are different, then there is an error.
The symbols Di and di are 8-bit values and the symbols Ei and ei are corresponding 10-bit encoded values. Symbols K0, and k0 are also 10-bit values, but are not necessarily encoded from 8 bit values. Examples of 8 to 10 bit encoding techniques include the well known 8b10b coding and transition minimized differential signaling (TMDS). An overview of TMDS is provided in chapter 3 of the Digital Visual Interface (DVI) revision 1.0 standard dated Apr. 2, 1999, available from the Digital Display Working Group at www.ddwg.org. DVI and High-Definition Multimedia Interface (“HDMI”) are each a TMDS-based signaling protocol.
Because the same transmitted test patterns should be received at RX under an error free environment, HSIO test often involves two identical PRBS 18 and 44 that are synchronized via a special control character(s), say K0, in the beginning of test pattern. The TX PRBS generates test patterns to transmit and the RX BIST generates an expected output to compare. The TX PRBS may generate the K0 and prefix it to the stream of codeword to be transmitted. When the designated control character (K0) is detected, the RX PRBS starts comparing expected test patterns with the received to identify failures. Test patterns in the PRBS are generated based on linear feedback shift registers (LFSR) 52. The behavior of LFSR 52 may be characterized by its characteristic polynomial. An example of LFSR 52 and its characteristic polynomial is shown in FIG. 2. The symbol ⊕ denotes an exclusive-OR (XOR) gate and the symbol D denotes a D-type flip flop, although other gates other than XOR gates may be used and flip flops other than D-type flip-flops may be used. The LFSR may generate a fixed sequence of test patterns. Its sequence is determined by the initial state called a seed.
The embedding fixed polynomial in RX PRBS may complicate the test for both internal and external customers. Some customers may want to use their own LFSR or ATE that may be equipped with the different polynomials. Their test applications may also require diversified test patterns from a number of different LFSR polynomials or shuffled version of existing LFSR generated test patterns.
The conventional PRBS often lacks online testing features. Online and off-line testing refers to tests runs during normal functional mode and test mode, respectively. Online testing capability may offer to measure channel quality with respect to bit-error-rate (BER) in real system applications. The PRBS is usually employed in an off-line test environment. It runs during test mode only and is usually disabled during a normal functional (normal operating) mode.